The present invention relates to control circuits for the switching of transistors, more particularly to a monolithically integratable control circuit for the controlled switching of power transistors, both of the bipolar and the field-effect type, for use in audio devices, for example.
Control circuits for the controlled switching of power transistors generally comprise a final stage of the push-pull type for the purpose of supplying to the load, with a high degree of efficiency, a flow of current that, at various times, can have opposite directions. In this way, high switching speeds can be achieved, accelerating, for example, the turn-off of transistors by extracting the charge stored therein.
FIG. 1 of the drawings shows a known circuit diagram of this type commonly used to control both bipolar and field-effect transistors.
The circuit comprises a first bipolar NPN transistor Q.sub.1, having a base terminal coupled by means of a first resistor R.sub.1 to a switching-signal source denoted in the figure by a block SW.
The emitter and collector terminals of this transistor are respectively connected to the negative terminal -V.sub.cc of a supply voltage generator and, by means of a constant-current generator A.sub.l, to the positive terminal +V.sub.cc of the supply voltage generator.
The circuit also comprises a second NPN bipolar transistor Q.sub.2 and a third NPN bipolar transistor Q.sub.3.
The base and emitter terminals of transistor Q.sub.2 are respectively connected to the collector terminal of transistor Q.sub.l and to the base terminal of transistor Q.sub.3.
The collector terminals of transistors Q.sub.2 and Q.sub.3 are both connected to the positive terminal +V.sub.cc.
The anode and the cathode of a first diode D.sub.l are respectively connected to the emitter and base terminals of transistor Q.sub.2.
The anode and the cathode of a second diode D.sub.2 are respectively connected to the emitter and the base terminals of transistor Q.sub.3.
A second resistor R.sub.2 is interposed between the emitter and base terminals of transistor Q.sub.3. The emitter terminal of transistor Q.sub.3 forms an output terminal OUT of the circuit.
Let us now examine the operation of the prior art circuit depicted in FIG. 1.
The switching signals supplied by the source SW give rise to transients of the voltage V.sub.IN applied to the base terminal of transistor Q.sub.1 through resistor R.sub.1 and with respect to the potential of the negative terminal -V.sub.cc may change from a "high" to a "low" level, and vice versa.
When the level of voltage V.sub.IN is high, transistor Q.sub.1 operates at saturation and absorbs all of the current supplied by generator A.sub.1, thereby keeping transistor Q.sub.2 turned off and consequently, transistor Q.sub.3 is kept off so as to not supply current to the load.
Since transistor Q.sub.3 is turned off, the level of the voltage V.sub.OUT at the output terminal with respect to the potential of the negative terminal -V.sub.cc is low.
Diodes D.sub.l and D.sub.2 permit the extraction of stored charge from the load connected to the output terminal during the turn-off phase. The load, which is not shown in FIG. 1, may be a power transistor.
When the level of the voltage V.sub.IN drops as a result of a switching signal originating in the source SW, transistor Q.sub.1 is turned off, so that all of the current supplied by the generator A.sub.1 is supplied to transistor Q.sub.2, which becomes conductive. Transistor Q.sub.3 as well, driven by transistor Q.sub.2, starts to conduct, supplying current to the load connected to the output terminal OUT. The level of the output voltage V.sub.OUT rises as soon as transistor Q.sub.3 leaves the OFF-state.
The constant-current generator A.sub.1 is so designed that it has an adequate flow of current not only to cause the swift conduction of transistors Q.sub.2 and Q.sub.3, which were previously turned off, but also to enable transistor Q.sub.1 to be switched rapidly from the saturation state to the OFF-state thanks to a rapid charge of the equivalent capacitance between collector and emitter of said transistor.